Cooperative Relay Scheme Having Backward Compatibility

ABSTRACT

In a relay scheme, a wireless source apparatus, a wireless destination apparatus and a wireless relay apparatus cooperate for handling transmission failures by space/time diverse channels. In the case of the successful direct transmission, reduced or no additional overhead for the relay selection is incurred. Thus, for a good SNR between source and destination, the inventive protocol has similar performance as a standard approach. In the case of a transmission failure e.g., due to small scale fading, a transmission via different communication paths implementing spatial diversity via a selected relay is supported. The device is to only activate the overhearing of signals in case of weak signal quality between sender and receiver. This selection of relay devices is done of demand only. A specific protocol for the reservation of the wireless medium for the entire cooperative communication has been specified.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from European Patent Application No.09014573.1, which was filed on Nov. 23, 2009, and is incorporated hereinin its entirety by reference.

BACKGROUND OF THE INVENTION

Mobile radio communications suffer from large-scale and small-scalefading effects that attenuate the communication signal. Whilelarge-scale fading is caused by distance-dependent path loss andshadowing effects, small-scale fading is caused by multipathpropagation. For mobile receivers or transmitters, small-scale fadingcan cause rapid fluctuations of the received signal-to-noise ratio(SNR); if a mobile device moves only a small distance it may experiencedeep fading even if it had perfect signal reception just an instantbefore.

Cooperative relaying [1] is a concept, where a relay node assists thecommunication between two nodes when the direct link is affected byfading. The information is relayed via a spatially different path whichis likely not affected by the same fading effects as the direct link atthe same time. Thus, using such a relay communication channel canimprove the communication performance by implementing spatial diversityfor the communication paths [12]. With the growing number of networkedwireless devices in everyday appliances, there are more potential relaynodes within transmission range of a sender and receiver. Henceforth,cooperative relaying will gain additional importance in the near future.

Cooperative diversity is expected to be more beneficial, if thecooperative relaying protocol is designed according to the following:First, it should have a low overhead, for example in terms of sentmessages, energy, storage, processing. A large number of communicationattempts are expected to succeed without the need for alternativecommunication paths. Thus, in the case of a successful transmission, acooperative relaying protocol should have minimal overhead in comparisonto non-cooperative transmission schemes. Second, the protocol shouldexploit cooperative diversity to an extent that makes the effort for themore complex interaction between wireless nodes worth it.

Cooperative relaying can be divided into three main phases: directtransmission, relay selection, and cooperative transmission. In thedirect transmission phase the source transmits its data, whereasdestination and relay (or potential relays) try to receive it. In therelay selection phase a neighboring node of source and destination isselected. The cooperative transmission phase, where the relay forwardsthe data to the destination, occurs only if the destination has failedto retrieve the data from the source during the direct transmission.

The relay selection phase has a great impact on the performance of thewhole cooperative relaying process [5]. The major selection criterion isthe link quality of the communication participants which is typicallymeasured by probing packets [6, 7, 10]. The selection can be furtherrefined by using additional factors like residual power [8]. Note thatthe selection process also depends on the actual environment, i.e., itis important to know how frequently a relay needs to be selected for agiven source destination pair, since a node may be a good relay at onetime instant and a bad one later on. In the absence of any environmentinformation, relays are selected for each packet anew [10]. Typically,relay selection is a distributed task which necessitates time and energyand thus introduces additional overhead. Therefore, it is beneficial toexplore the current realization of the channel between source anddestination and to do relay selection only on demand [9].

Relay selection can be done before direct transmission (proactive relayselection) or after direct transmission (reactive relay selection).Proactive relay selection is considered to have energy advantages overreactive relay selection since only the selected relay needs to spendenergy for listening to the transmission of the source [10]. However, itintroduces a constant overhead to all transmissions. Moreover, thechannel state may change during the direct transmission phase. Theselected relay might then not be able to receive the data, or the relaylink to the destination can also change considerably making successfulrelaying unlikely. In reactive schemes, a relay is selected only if thedestination is not able to receive the data from the source and asks forassistance (cf. iterative relaying [12]). Thus, reactive relay selectionis only done if a direct transmission fails and relaying candidates havealready received the original data from the source properly. Adisadvantage is that all potential relaying nodes need to listen to thetransmission of the source. Since many transceivers consume the sameorder of energy for receiving and transmitting [11], the costs forhaving all neighbors of source and destination listen might not benegligible.

Recently, cooperative relaying is no longer treated as a separate taskbut is investigated in combination with MAC protocols. It is beneficialto exploit channel reservation messages such as Request-To-Send (RTS)and Clear-To-Send (CTS) for probing the channel and selecting a relay[13, 14], since relays also need to make a channel reservation for thecooperative transmission phase. The herein proposed approach selectsrelays after the direct data transmission phase and thus, does notintroduce additional delay to successful direct transmissions. In [13]neighbors determine whether cooperation is necessitated based on thechannel quality between source and destination. Relay selection and therelay's channel reservation are done by using a slotted approach beforethe direct transmission.

Liu et al. introduce in [14] a protocol which uses information from thepast, to determine whether the data transfer rate between two nodes canbe improved by using a relaying node. Channel reservation for source,destination, and relay are done before direct transmission.

Moh et al. uses in [15] pro-active relay selection based on historicaldata and thus favor nearby neighbors as relay. Their protocol extendsCSMA/CA to use distributed standard block codes (D-STBC) when needed,which allows the simultaneously transmission from two or more nodes. Thecooperation is not just used for data transfer but also for signallingmessages like RTS and CTS.

In summary, disadvantages of known relaying protocols are lowflexibility, waste of resources including transmissionresources/bandwidth on the one hand and battery power of wirelessdevices on the other hand due to the overhearing/storing of messages inrelay devices, which would not have been necessitated.

SUMMARY

According to an embodiment, a wireless relay apparatus for receiving asource data message and for transmitting a relay data message based onthe source data message may have: a transmitter section and a receiversection; and a controller configured for determining, whether thewireless relay apparatus will, when operating as a wireless relayapparatus, provide a transmission channel from a wireless sourceapparatus to a wireless destination apparatus via the wireless relayapparatus, which is better than a direct transmission channel from thewireless source apparatus to the wireless destination apparatus or fordetermining, whether the wireless destination apparatus necessitates arelay operation or does not necessitate a relay operation; configuredfor storing the source data message from a wireless source apparatusonly when the transmission channel via the wireless relay apparatus isbetter than the direct transmission channel or when a relay operation isnecessitated, and for not storing the source data message in the othercase; and configured for controlling the transmitter to transmit therelay data message, the relay data message being based on the storedsource data message, wherein a cooperative clear to send messagereceived includes a channel condition estimate on the directtransmission channel between the wireless source apparatus and thewireless destination apparatus, and wherein the controller is configuredfor extracting the channel condition estimate from the cooperative clearto send message received to obtain a direct channel information, whereinthe controller is configured for determining a relay channel informationfor the transmission channel from the wireless source apparatus via thewireless relay apparatus to the wireless destination apparatus, andwherein the controller is configured for comparing the direct channelinformation and the relay channel information to determine, whether thewireless relay apparatus will provide a better transmission channel.

According to another embodiment, a wireless destination apparatus forreceiving a data message may have: a transmitter section and a receiversection; and a controller configured for interpreting messages receivedby the receiver section; configured for estimating a channel conditionbetween a wireless source apparatus and the wireless destinationapparatus in response to a request to send message originating from thewireless source apparatus; configured for controlling the transmittersection to transmit a clear to send message when the channel conditionis good enough without a relaying operation and to transmit acooperative clear to send message when the channel condition is so thata relaying operation will become necessitated, wherein the controller isconfigured to form the cooperative clear to send message so that achannel condition estimate is included in the cooperative clear to sendmessage, wherein the controller is in a relay mode, when the cooperativeclear to send message is transmitted and is in a non-relay mode, whenthe clear to send message is transmitted, and wherein the controller isconfigured to control the transmitter section to transmit, in thenon-relay mode, an acknowledge message subsequent to transmitting aclear to send message and a successful receipt of a source data message,and to transmit, in the relay mode, a negative acknowledge messagesubsequent to transmitting a cooperative clear to send message and anon-successful receipt of the source data message.

According to another embodiment, a method of operating a wireless relayapparatus for receiving a source data message and for transmitting arelay data message based on the source data message, including atransmitter section and a receiver section; and a controller, may havethe steps of: determining, whether the wireless relay apparatus will,when operating as a wireless relay apparatus, provide a transmissionchannel from a wireless source apparatus to a wireless destinationapparatus via the wireless relay apparatus, which is better than adirect transmission channel from the wireless source apparatus to thewireless destination apparatus or determining, whether the wirelessdestination apparatus necessitates a relay operation or does notnecessitate a relay operation; storing the source data message from awireless source apparatus only when the transmission channel via thewireless relay apparatus is better than the direct transmission channelor when a relay operation is necessitated, and not storing the sourcedata message in the other case; and controlling the transmitter totransmit the relay data message, the relay data message being based onthe stored source data message, wherein a cooperative clear to sendmessage received includes a channel condition estimate on the directtransmission channel between the wireless source apparatus and thewireless destination apparatus, and wherein the channel conditionestimate is extracted from the cooperative clear to send messagereceived to obtain a direct channel information, wherein a relay channelinformation for the transmission channel from the wireless sourceapparatus via the wireless relay apparatus to the wireless destinationapparatus is determined, and wherein the direct channel information andthe relay channel information are compared to determine, whether thewireless relay apparatus will provide a better transmission channel.

According to another embodiment, a method of operating a wirelessdestination apparatus for receiving a data message, including atransmitter section and a receiver section; and a controller, may havethe steps of: interpreting messages received by the receiver section;estimating a channel condition between a wireless source apparatus andthe wireless destination apparatus in response to a request to sendmessage originating from the wireless source apparatus; controlling thetransmitter section to transmit a clear to send message when the channelcondition is good enough without a relaying operation and transmitting acooperative clear to send message when the channel condition is so thata relaying operation will become necessitated, wherein, when the clearto send message is transmitted, an acknowledge message is transmittedsubsequent to transmitting the clear to send message and a successfulreceipt of a source data message, and wherein, when the cooperativeclear to send message is transmitted, a negative acknowledge message istransmitted subsequent to transmitting the cooperative clear to sendmessage and a non-successful receipt of the source data message.

Another embodiment may have a computer program for performing, whenrunning on a computer, the inventive methods.

The present invention is based on the finding that an efficient andflexible concept of cooperative relaying is obtained by having awireless source apparatus performing an extended channel reservationsubsequent to a transmission of the source data message. This extendedchannel reservation is only activated, when the wireless destinationapparatus transmits a negative acknowledge message subsequent to atransmission of the data message by this wireless source apparatus.

A further aspect is that the wireless source apparatus transmits thesource data message without any delay irrespective of, whether acooperative relay is necessitated or not. Therefore, the data messagetransmission is not delayed until e.g. the best relaying candidate hasbeen identified and selected.

The extend channel reservation (ECR) message transmitted/initiated bythe wireless source apparatus makes sure that the wireless sourceapparatus, although already having transmitted the source data messagehas control over the channel reservation. The channel reservation isvalid for devices in the neighborhood of the wireless source devicecurrently not contributing to the cooperative relay scheme. Hence, it ismade sure that wireless devices in the neighborhood of wireless sourcedevices are efficiently blocked and do not interfere with the sourcedevices' ability to receive an acknowledgement message from thedestination. When a cooperative relay process is necessitated, then thisacknowledgement message will be received after the whole relayingprocess.

In a further aspect, this ECR message is useful for triggering acontention procedure among several wireless relay apparatuses, when morethan one potential relay apparatus candidate exists. In response toreceiving the ECR message, the potential wireless relay apparatus isconfigured for initiating the contention procedures so that, in the end,a wireless relay apparatus surviving the contention procedure willtransmit an apply for a relay (AFR) message or will transmit the datamessage itself, depending on the relay candidate selection algorithmimplemented.

In a further aspect, the wireless destination apparatus cooperating withthe wireless relay apparatus and/or the wireless source apparatusperforms a destination-driven selection process for selecting a usefulwireless relay apparatus among a plurality of wireless relay apparatuscandidates. This procedure follows the transmission of a negativeacknowledgement message from the wireless destination apparatus.Subsequent to the transmission of this negative acknowledgement message,the wireless destination apparatus will, in the end, transmit a selectfor relay (SFR) message identifying the selected wireless relayapparatus which is to perform the relay action, i.e., to transmit therelay data message to the wireless destination apparatus, so that thiswireless destination apparatus can, in the end, transmit anacknowledgement message.

In a further aspect, the battery power consumed by overhearing/storingsource data messages by potential relay candidates is effectivelyreduced. To this end, wireless relay apparatus candidates becoming awareof an upcoming communication between a wireless source apparatus and awireless destination apparatus do not automatically store the sourcedata message. Instead, in this aspect, the potential wireless relayapparatus candidate actually determines, as far as possible, by therelay apparatus whether a relaying operation involving the actualwireless relay apparatus candidate will, in fact, become necessitated atall or, provided that it will become necessitated, will actually bebetter than the direct source destination channel.

In a further aspect, a wireless destination apparatus, when beingaddressed by a request to send message from a wireless source apparatusestimates the channel condition between the wireless source apparatusand the wireless destination apparatus. Based on the determined channelcondition, the wireless destination apparatus either sends a CTSmessage, when the channel is good enough, or sends a cooperative clearto send (CCTS) message indicating two potential neighboring relaycandidates that a relaying operation will become necessitated. Thisfeature enhances efficiency and backward compatibility due to the factthat it is avoided to perform a cooperative relaying operation. Instead,a cooperative relaying operation is only initiated when it is actuallyneeded. When, however, the destination determines that a relayingoperation is not necessitated, since the transmission channel for thedirect transmission between source and destination is good enough, astraightforward clear to send message is transmitted which informs allneighboring devices that they do not have to spend any further bandwidthor battery power for being ready for a relaying operation, which will,however, never come due to the clear to send message rather than theCCTS message.

A further embodiment relates to a method or apparatus or computerprogram for operating a wireless destination apparatus for receiving adata message, comprising a transmitter section and a receiver section;and a controller, comprising: interpreting messages received by thereceiver section; estimating a channel condition between a wirelesssource apparatus and the wireless destination apparatus in response to arequest to send message originating from the wireless source apparatus;controlling the transmitter section to transmit a clear to send messagewhen the channel condition exceeds a given threshold and to transmit acooperative clear to send message when the channel condition does notexceed a given threshold.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will be detailed subsequentlyreferring to the appended drawings, in which:

FIG. 1 a is a flowchart illustrating actions performed by a wirelesssource device;

FIG. 1 b is a flowchart illustrating actions of a wireless destinationapparatus;

FIG. 1 c is a flowchart illustrating actions of a wireless relayapparatus;

FIG. 2 a is a diagram illustrating the functionality of a wirelesssource apparatus in accordance with one aspect;

FIG. 2 b is a diagram illustrating the procedure performed by a wirelessdestination apparatus in accordance with a further aspect;

FIG. 2 c is a flowchart illustrating operations performed by a wirelessdestination apparatus in accordance with a further aspect;

FIG. 2 d is a flowchart illustrating operations performed by a potentialwireless relay apparatus in accordance with a further aspect;

FIG. 2 e is the continuation of the procedure illustrated in FIG. 2 d;

FIG. 2 f is an illustration of operations performed by a wireless relayapparatus in accordance with a contention protocol;

FIG. 3 is a diagram generally illustrating a cooperative relayingsituation;

FIG. 4 is a packet exchange and channel reservation diagram;

FIG. 5 is a list of signaling packets/messages;

FIG. 6 is a table illustrating simulation parameters for the simulationsillustrated in FIGS. 7 to 10;

FIGS. 7-10 are different simulation results of an implementation; and

FIG. 11 illustrates a wireless mobile apparatus usable as a wirelesssource apparatus, a wireless destination apparatus, an active wirelessrelay apparatus or a neighboring wireless apparatus not participating ina current cooperative relaying scheme.

DETAILED DESCRIPTION OF THE INVENTION

The invention relates to an innovative concept for cooperative relayingin wireless systems. If a signal transmitted from a sender device to areceiver device cannot be correctly received at the receiver device, theproposed method selects a relay device which could correctly receive thesignal beforehand. The invention is advantageous over existing methodswith respect to the following individual aspects: (1) Relay devices doonly activate the overhearing of signals in case of weak signal qualitybetween sender and receiver. (2) The selection of relay devices is doneon demand only. (3) A specific process (protocol) for the reservation ofthe wireless medium for the entire cooperative communication has beenspecified.

A networked system consisting of a number of devices which are able tocommunicate with each other by wireless radio is assumed. The intendedservice is to deliver information from one network node, the source, toanother network node, the destination.

Wireless links typically suffer from fading causing degradations of thecommunication channel. These fading effects on a channel may change overtime and vary with the spatial position of a node. In cooperativerelaying, an additional communication path is added via relaying nodes.Due to the spatial diversity, it can be assumed that the communicationpaths via relays have fading effects which are sufficiently uncorrelatedto the fading problems on the original path. By selecting thecommunication path with the currently best physical properties, theinformation can be transmitted.

In accordance with aspects of the invention, at least one transmitterand one receiver are actively engaged in each communication. All othernodes may act as potential relay stations if, (1) they have a link tothe transmitter as well as to the receiver, and, (2) the channelreservation response from the receiver indicates a bad channel statebetween transmitter and receiver (selection between potential relayingor non-relaying). All potential relays try to receive the packetbroadcasted from the transmitter. If the receiver can decode the packet,the direct transmission has succeeded without the need for relaying. Ifthe receiver cannot decode the packet, it starts a relay selection phasethat involves all relays which have received the packet from thetransmitter correctly. The relay selection phase involves an innovativeprotocol, which selects the relay with the best relay-destinationchannel within a defined time frame with high probability. The selectedrelay station transmit the packet to the receiver (cooperativetransmission).

FIG. 3 depicts an example of cooperative relaying. The source node Stries to transmit a message to the destination D (see communication (1)in the figure). The signal strength of communication (1) is sketched bycircles in different grayscale colors. Note that the particular signalstrength and SNR ratio typically are not homogenous as indicated in thefigure but may change significantly at a small scale. Nevertheless, thissimple example is useful to depict the approach. It is assumed that thetransmission does not reach the destination node with a signal strengththat allows for a correct/complete decoding of the original message.Concurrently with communication (1), the message was overheard by node R(communication (2)). After that, node R forwards this information to thedestination (communication (3)). Since the communication (2) and (3)took a different path than communication (1), this example implementsspatial diversity. Since communication (1) and (3) are preformedsubsequently, the presented approach also contains the concept of timediversity.

As long as messages can be overheard by potential relays and the relaysexperience uncorrelated fading effects—both of these assumptions holdfor typical wireless systems—the cooperative relaying approach iseffective for increasing the reliability and/or network throughput. Theapproach is also efficient in terms of number of sending operations,since (1) and (2) originate from a single broadcast operation. FIG. 3additionally illustrates a neighboring node N which, for example,overhears the message from the source device S, but can, for example,not contribute to the source destination connection, so that theconnection is improved by the relaying operation. In one embodiment, asdescribed later, this neighboring node N determines that it cannotcontribute, so that this neighboring node does not waste resources bystoring messages from the source device.

MAC changes considerably in the presence of cooperative relaying. Innon-cooperative schemes, the wireless medium is reserved just in theneighborhood of source and destination for the time of the directtransmission and the acknowledgment. In cooperative relaying, however,the channel reservation needs to be extended in space and time for therelaying. This leads to reduction of the spatial re-usability of thenetwork since the channel reservations for the relay might interfere orblock other communications which otherwise could be done concurrently ifthe relay is not used. Furthermore, in proactive relay selection, relaysare selected and the channel floor for them are reserved before directtransmissions. Whenever the direct transmission succeeds, thosereservations block other communications and degrade the overallthroughput.

An advantageous inventive aspect is that the system is efficient incases when relaying is not needed. Thus, the introduced overhead bycooperative relaying is minimized. The system follows a reactive relayselection approach such that it behaves similar to standardnon-cooperative protocols in the absence of link errors on the directlink in terms of throughput. Besides reducing the introduced overheadwhen cooperation is not needed, reactive relay selection also inherentlyprioritizes direct transmission with respect to cooperative ones.Inventive aspects also address the higher energy consumption of reactiveschemes by determining whether cooperation can be used or not beforedirect transmission and by limiting the number of nodes listening to thedata transmission between source and destination.

Four different aspects of the present invention are subsequentlydescribed in more detail with respect to FIGS. 1 a to 2 f, the completepackage exchange and general reservation schedule of an embodiment ofthe present invention is described in the context of FIG. 4. FIG. 4indicates time running from left to right and illustrates, under letter(a) a straightforward efficient procedure without a cooperation request.Under letter (b), the time sequence of commands sent by source,destination and different relay candidates is illustrated

Hereafter we referred to the channel between source-destination,source-relay, and relay-destination as SD-channel, SR-channel, andRD-channel, respectively. The multi-hop channel between source anddestination via the relaying node is addressed by SRD-channel.

FIG. 4 depicts channel allocations and packet exchange for successfuland failed direct transmissions and FIG. 5 summarizes the names and thesize of the used signaling packets of our protocol. Packets markedwith * are modified versions of standard CSMA/CA packets or newlyintroduced ones. The length of the packets has been chosen in compliancewith IEEE 802.11. Signaling packets are sent using a lower modulationscheme that is slower, but less prone to transmission errors than themodulation of the data message. Dark bars in FIG. 4 indicate channelreservations of a particular node.

A communication attempt starts with an RTS transmission from the sourcewhich is used to inform the intended destination about the duration ofthe data transmission and reserves the channel floor until the responseof the destination is received. At reception of an RTS, the destinationdecides whether cooperation is needed or not (see section 3.4). If thedestination does not reply (i.e., it has not received the request or itis not allowed to answer due to another communication), a back off likein CSMA/CA is performed. Each unanswered RTS increases the small messageretry counter. If this counter exceeds a maximum value the data isdropped. In FIG. 4 a we show the message exchange and channelreservation for the case where no cooperation is requested. Thedestination replies with a CTS message, which reserves the channel forthe data transmission of the source. When the source receives the CTSmessage it starts transmitting the data message. The data messageextends the channel reservation in the vicinity of the source untilreception of an Acknowledge (ACK) message from the destination. Note,that there is no additional delay introduced compared to CSMA/CA. If thedirect transmission fails, the large message retry counter isincremented. Until the value of this counter exceeds a specified maximumvalue, the source tries to occupy the channel for a retransmission.Otherwise the data message is dropped.

The message exchanges and the channel reservations if the destinationdecides to use cooperation are depicted in FIG. 4 b. The destinationuses a CCTS message, which contains the information of a CTS messageextended by the SNR value of the SD-channel, to inform source andneighbors that cooperation is necessitated. The channel reservation of aCCTS lasts until the end of the data transmission. The reception of theCCTS triggers the same actions at the source as a CTS. Thus, if thedirect transmission succeeds, there is no additional overhead incomparison to CSMA/CA, except for a 14% transmission time increase forthe CCTS instead of CTS, which is not significant since the controlmessage itself only accounts for a small part of the overall messagetransmission time.

When the direct transmission fails the destination uses aNegative-Acknowledge (NACK) message to inform source and relayingcandidates that a cooperative transmission is necessitated. The NACKextends the channel reservation of the destination until the end of therelay selection phase. Since the source needs to know the success of thecooperative transmission, it extends its own channel reservation untilthe expected ACK reception by broadcasting an Extend-Channel-Reservation(ECR) message. After this message the slotted relay selection processstarts where relaying candidates use Apply-For-Relay (AFR) messages toindicate their relaying readiness. These messages are also used toreserve the channel floor until the end of the relay selection phase.Reservations in the relay selection phase (i.e., those done by NACK,ECR, and AFR messages) only block nodes which are not participating inthis cooperative transmission attempt to access the channel, e.g.,relaying candidates are not prevented from sending their AFR messages.At the end of the relay selection phase the destination transmits an SFRmessage which selects the current relay and also extends the channelreservation until the end of the data transmission from the relay. Atreception of the SFR the selected relay starts transmitting theoverheard data from the source. Finally, the destination signals thesuccess of the cooperation. If the cooperation attempt is notsuccessful, retransmission from the source is invoked. In theretransmission phase the channel reservation process in FIG. 4 isrepeated.

FIG. 11 illustrates a wireless mobile device. The wireless mobile devicecan be implemented as a wireless source apparatus, a wirelessdestination apparatus or a wireless relay apparatus. A wireless mobiledevice is implemented to be in the position to fulfill all threefunctionalities, i.e., the functionality of a source, of a destinationor a relay in accordance with the inventive aspect. However, this is notnecessarily the case, since a device that can only act as a sourcedevice or a destination device or a relay device will already incurimprovements.

The wireless apparatus illustrated in FIG. 11 at 100 comprises acontroller 102 and a transmitter section and receiver section 104, whichis also named to be a TX/RX front end. The controller 102 may compriseknown elements, such as a microprocessor, volatile storage, non-volatilestorage, interfaces and other elements necessitated to provide thefunctionality of a mobile device and, particularly, to provide thesequence of steps in accordance with aspects of the protocol illustratedin FIG. 4.

In a flowchart for a wireless source device implementing the procedureillustrated in FIG. 4, the controller will make sure that the sourceapparatus sends an RTS message 110. The request to send message willcomprise, among others, at least the identification of the intendeddestination apparatus. Additionally, the request to send message maycomprise data on the amount or length of a data message to be sent bythe wireless source apparatus.

In the case of the cooperative relay situation, the wireless sourceapparatus will receive, in a subsequent step, a CCTS message 112.Basically, the controller 102 in FIG. 11 will monitor messages incomingover the transmitter/receiver section 104. In accordance with aspecified protocol, the controller is configured for interpretingincoming messages and, as soon as the controller interprets an incomingCCTS message having an identification of the wireless source device orany other link to the RTS message sent in step 110, the controller ofthe wireless source apparatus will go over to step 114. In step 114, thedata message, i.e., the source data message is sent out via thetransmitter/receiver section 102 in FIG. 11. Then, the subsequent step116 of the wireless source apparatus will be to receive a negativeacknowledge message (NACK) from the intended wireless destinationdevice. This is not surprising, since the wireless destination apparatushas already sent a CCTS message before, indicating that the directchannel between source and destination is not good enough.

Subsequent to the reception of the NACK message, the source apparatuswill send out an ECR message 118, making sure that all neighboringdevices in the neighborhood of the wireless source apparatus are blockedfrom using the reserved channel resources. This allows the wirelesssource apparatus to be in the position to listen to a finalacknowledgement from the wireless destination apparatus, which thewireless destination apparatus will send out as soon as the relayingprocess was successful, i.e., the relay data message has beensuccessfully transmitted from a relay apparatus to the destinationapparatus. Hence, the relaying process is terminated by transmitting theacknowledgement message from the wireless destination apparatus, and,with respect to the wireless source apparatus illustrated in FIG. 1 a,the relaying process is terminated by receiving the acknowledgementmessage as indicated at 120 in FIG. 1 a. In response to the receipt ofthe acknowledgement message from the wireless destination apparatus, thewireless source apparatus is in the position to start a new datatransmission by issuing a new RTS message. Naturally, the wirelesssource apparatus can change its role so that, in the next cycle, thewireless source apparatus can become a destination apparatus or a relayapparatus, when the mobile device according to which the FIG. 1 aprocedure is implemented, does not only have the functionality of awireless source apparatus, but also has the functionality of a wirelessdestination apparatus or a wireless relay apparatus.

FIG. 1 b illustrates the procedure performed within a wirelessdestination apparatus, where the sequence of steps is implemented by thefunctionality of the controller 102, illustrated in FIG. 11.

The destination apparatus starts its process by receiving an RTS message122 from a wireless source device. The RTS message includes, amongothers, an identification of the wireless destination apparatus. In step122, the wireless destination apparatus determines whether a cooperationis needed, or whether a direct channel is good enough for a successfultransmission of the source data message. When step 124 results in adecision that a cooperation is not necessitated, the destinationapparatus outputs a CTS message and the procedure proceeds asillustrated in FIG. 4 a. Then, a relaying process is not necessitated atall and a straightforward direct transmission is performed, which is themost efficient and most rapid data transmission. When, however, it isdetermined by the destination apparatus that a cooperation is needed,the actual source destination channel is determined as indicated at 126.To this end, the destination apparatus determines a general estimate ofthe actual SD channel which is an SNR value of the direct source drainchannel as determined by the reception of the RTS message from thewireless source apparatus.

In step 128, the wireless destination apparatus issues a CCTS messagewhich includes the source destination channel information as determinedin step 126. In step 130 the controller in the wireless apparatusdetermines, whether the direct transmission has failed. When the directtransmission has not failed, the wireless destination apparatus wouldissue an acknowledgment (ACK) message and a cooperation operation wouldnot be performed although initiated in step 124. The more probable case,however, is that step 130 results in the outcome that the directtransmission of the source data message to the destination has failed.This means, in other words, that a bit error rate or a packet error rateis higher than an admissible threshold. In this case, the wirelessdestination apparatus proceeds with step 132, i.e., by sending anegative acknowledge message NACK. Now, a relay contention protocol isstarted. This results in the reception of one or more AFR messages 134depending on the number of potential relay candidates. In step 136, thewireless destination apparatus selects one of the available relaycandidates and selects the best relay candidate. The best relaycandidate is the candidate which has the best signal to noise ratio withrespect to its AFR message. In other words, the destination apparatusdetermines the signal to noise ratio associated with the AFR message,and then sends an SFR message to the best relay apparatus 138. To thisend, the AFR message comprises an identification of the apparatus, fromwhich the AFR message originates, and the SFR message from thedestination comprises an identification of the wireless apparatus, whichhas sent the corresponding AFR message.

As soon as the wireless destination apparatus has transmitted the SFRmessage to select the best relay node, the relay node will start sendingthe stored data message, and this stored data message is received instep 140. As soon as the wireless destination apparatus has successfullyreceived the data message, the wireless destination apparatus sends outan acknowledgement message 142. Then, the relaying process issuccessfully completed and a new cycle can start. Again, the wirelessdestination apparatus can change its role in the new cycle so that thewireless destination apparatus operates in the new cycle as a wirelesssource apparatus or a wireless relay apparatus.

FIG. 1 c illustrates the procedure performed by a wireless relayapparatus in accordance with an embodiment. The relay apparatus receivesoverheard CCTS and RTS messages in step 144. In step 146, the wirelessrelay apparatus calculates the channel of a transmission path betweenthe wireless source apparatus, the wireless relay apparatus and thewireless destination apparatus. This relay channel estimation isperformed by calculating the SNR associated with the reception of theCCTS message and the SNR associated with the reception of an RTSmessage. Naturally, the SNR values are determined for correspondingRTS/CCTS pairs, i.e., for a CCTS message from a wireless deviceidentified by the preceding RTS message. The relay channel informationis calculated as a packet error rate value as indicated in step 146. Instep 148, the wireless relay apparatus extracts a packet error rate fromthe source for the destination, i.e., for the direct channel. This valuecannot be calculated by the relay apparatus itself, since the relayapparatus cannot be aware of the direct channel. Therefore, the relayapparatus extracts the value of the direct channel from the receivedCCTS message. Then, the direct channel can be compared to the relaychannel by the wireless relay apparatus. When the direct channel isbetter than the relay channel, then the relay device determines that arelaying over this relay device does not result in any improvement. Inthis case, the relay apparatus does not store the overheard data messageas indicated at 150, and the relay apparatus does not engage in anyrelay contention protocol. Furthermore, the relay apparatus has savedconsiderable resources with respect to memory and battery power due tothe fact that it has not stored the data message. In a step 152, therelay apparatus overhears NACK, ECR or AFR messages and understandsthese messages so that the apparatus illustrated in FIG. 1 c will notuse resources reserved by these messages in order to not disturb therelay contention operation and the final data transmission of adifferent relay node which can actually improve the direct channel fromsource to destination as indicated in step 154.

When however, the relay device has determined that it will improve thetransmission situation, e.g., when the packet error rate of the relaychannel is smaller than the packet error rate of the direct channel, therelay apparatus receives and stores the data message transmitted by thesource apparatus as indicated at 156. The next event experienced by therelay apparatus will be the reception of an NACK message 158 and thereception of an ECR message 160. Subsequent to either the NACK messageor the ECR message or the sequence of the messages, the relay apparatusselects a transmission resource for the to be transmitted AFR message162. The selection of the necessitated data transmission resource suchas a time slot or a frequency carrier or a code channel or a spatialchannel is performed by a certain contention algorithm. Subsequent tothe selection of a certain resource such as a time slot, the relayapparatus transmits an AFR message which has an identification of thiswireless relay apparatus. When the relay apparatus is selected to be theactual relaying device, the relay apparatus receives an SFR messagehaving an identification of this wireless relay apparatus as indicatedat 166. When, however, the wireless destination apparatus has selected adifferent relay apparatus, the relay apparatus does not output any data.When, however, the relay apparatus has been selected, the next step isthe transmission of the stored data message 168, As soon as the datamessage is transmitted the general reservation with respect to the relayapparatus is cancelled and the relay apparatus is ready for otheractions. As indicated in FIG. 4 b, the relay apparati which have notbeen selected by the SFR message reset their protocol state and becomeavailable for other communications in the network.

FIG. 2 a illustrates a wireless source apparatus for transmitting asource data message which performs a specific NACK-ECR-ACK sequence. Inaccordance with this embodiment, the wireless source apparatus comprisesa transmitter section and a receiver section. Furthermore, the wirelesssource device comprises a controller configured for controlling thetransmitter section to transmit a source data message illustrated at 200in FIG. 2 a. Subsequent to the transmission of the data message, thecontroller is configured for interpreting incoming messages received bythe receiver section. When a message received by the receiver section isinterpreted to be an negative acknowledgment message as indicated at202, the controller is configured for controlling the transmittersection to transmit an ECR message 204. This ECR message is specificallytransmitted when a message received by the receiver section isinterpreted to be a negative acknowledgement message from a wirelessapparatus being an intended destination of the source data message. Thecontroller is configured for setting the wireless source apparatus in await mode 206, in which the wireless source apparatus is ready toreceive an ACK message from the intended wireless destination apparatus.As soon as the ACK message is received, the wireless source device isready for a new cycle or a new data message depending on the actualfunctionality/role of the wireless devices in the next procedure.

Specifically, it is to be outlined that all neighboring nodes but thedestination node and actually useful relay nodes remain silent inresponse to the ECR message as indicated at 210.

Thus, specifically the features of the FIG. 2 a embodiment are that thesource transmits a reservation message ECR which effectively blocksnodes being positioned close to the source device for using criticaltransmission resources. As illustrated in FIG. 4 b, the same is done fornodes in the neighborhood of the wireless destination apparatus, sincethese devices interpret the NACK message of the destination apparatus asa channel reservation and the only wireless relay devices which are notblocked by this channel reservation are the wireless relay apparatuscandidates which have determined by themselves that they can positivelycontribute to the data transmission between the source device and thedestination device.

It is a specific feature of the FIG. 2 a embodiment that the reservationmessage ECR is transmitted by the source rather than any othercontributing party. Furthermore, it is advantageous that the ECR messagehas timing information at least on the length of the following datamessage, which of course, is known to the source of the data.Furthermore, the ECR message additionally comprises information on thetime length of the contention phase following the ECR message.

Additionally, it is to be noted that the length of the reservationinitiated by the ECR message is in one embodiment, based on thepredefined time setting or is determined by timing information includedin the ECR message or actually lasts until the receipt of the ACKmessage from the destination. Then, all channel reservations can becancelled by all devices which had any channel reservations before,since the transmission operation has been successfully completed.

FIG. 2 b illustrates an additional embodiment of a wireless destinationapparatus for receiving a data message. Again, the wireless destinationapparatus comprises a transmitter section and a receiver section such asitem 104 of FIG. 11 and a controller such as 102 in FIG. 11. Thecontroller is configured for interpreting messages received by thereceiving section and is configured for estimating a channel conditionbetween a wireless source apparatus and a wireless destination apparatus220 in response to an RTS message originating from the wireless sourceapparatus 222. In a step 224, the controller compares the channelinformation for the direct channel to a specified threshold. When thethreshold is exceeded, this means that a relay/cooperation isnecessitated so that in this instance, step 226 determines an SNR basedon the reception of the RTS message, or generally, uses the channelinformation determined in step 220. In step 228, the destinationapparatus sends a CCTS message, which is similar to a CTS message apartfrom an additional SNR estimate or generally, a channel information.Subsequent to sending the CCTS message, one can say that the controlleris in a relay mode. When however, it is determined in step 224 that thechannel information such as a packet error rate PER or bit error rateBER is below the threshold, steps 226 and 228 are not performed, andinstead, a straightforward CTS message is sent 230, which means that thenormal efficient process without relay/cooperation is used whenpossible, and which additionally means, that the more efficient relaymode is only used when necessitated, and is for example not used all thetime which would highly contribute to a waste of battery power,transmission resources etc. Although a determination between the nonrelay mode and the relay mode is done depending on whether channelinformation exceeds or is below a threshold, it is clear that, dependingon the actually channel, the situation can be vice versa, i.e., that acertain threshold has to be exceeded to go into the non-relay mode, anda value has to be below the threshold in order to go into the relaymode. Generally, the non-relay mode is selected when the generalinformation determined in step 220 has a first relation with respect tothe threshold, and the relay mode is selected when the channelinformation has a second relation to a threshold which is different fromthe first threshold, and which is the opposite of the first relation.

FIG. 2 c illustrates a wireless destination apparatus for receiving adata message, which again comprises a transmitter section and a receiversection and a controller as illustrated in the context of FIG. 11. Thecontroller is configured for interpreting messages received by thereceiver section, and is configured for controlling the transmitter tosend a negative acknowledge message 240 in response to a non-successfulreception of a data message from a wireless source apparatus asindicated at 242. In an embodiment, the decision in step 242 can bepreceded by the transmission of a CCTS message 244, but in otherembodiments, functionality of steps 242, 240 can be invoked as well.When step 242 determines that the data message has been receivedproperly, an ACK message is sent by the destination device as indicatedat 246. This means that a full relaying process is not initiated,although a CCTS message has been sent before. This procedure is usefulin the case where the determination in step 242 was too pessimistic, orwhen the channel is not stationary. This is a specific feature of theembodiment in that there remains a chance for a successful directtransmission, since the wireless source apparatus transmits the datamessage after receiving a CTS or a CCTS message irrespective of whetheran actual relaying selection has already been done or not. At the timeinstant, where the source device sends out the data message, althoughhaving received a CCTS message from the destination apparatus in FIG. 2c, the source does not know that a relaying channel has beenconstructed. However, due to the inventive protocol, nothing gets lostand an efficient relay candidate selection takes place, which does nothave any effect on the source device apart from the fact that the sourcedevice has to wait for a final or immediate acknowledgement from thedestination.

Subsequent to the transmission of an NACK message in step 240, thecontention procedure 248 takes place, which may consume a certainpredefined time period as determined or as actually fixed in thedestination apparatus. Thus, the controller of FIG. 11 is configured forcontrolling the transmitter section to send an SFR message 250identifying selected wireless relay apparatus subsequent to sending theNACK message 240, when at least one AFR message from a wirelessapparatus is received in the time period consumed by the contentionprocedure 240, which precedes the time of sending the SFR message 250.Finally, the transmitter section is controlled to send anacknowledgement message subsequent to a successful reception of the datamessage from a selected wireless relay apparatus, where this selectionhas taken place by the SFR message sent in step 250.

The specific feature of this aspect is the NACK-SFR sequence, whichmeans that the NACK message provides for an efficient channelreservation in the neighborhood of the wireless destination apparatus.The destination apparatus has the possibility to receive AFR messagesfrom potential relay candidates, and that the contents and procedure iscontrollably terminated by the destination device itself, i.e., bysending the SFR message 250. Thus, the destination initiates thecontention selection of relay nodes, and the source device does not haveto care about all this contention and more importantly, does not have towait for a successful relay selection, but can simply transmit the dataimmediately after receipt of a CCTS message.

FIG. 2 d illustrates an embodiment of a wireless relay apparatus forreceiving a source data message and for transmitting a relay datamessage based on the source data message. Again, the relay apparatuscomprises a transmitter section and a receiver section and a controlleras discussed in connection with FIG. 11. The controller is configuredfor determining whether the wireless relay apparatus will, whenoperating as a wireless relay apparatus, provide a transmission channeland a wireless source apparatus to a wireless destination apparatus viathe wireless relay apparatus which is better than a direct transmissionchannel from the wireless source apparatus to the wireless destinationapparatus as indicated in step 260, or for determining whether awireless destination apparatus actually necessitates a relay operationor does not necessitate a relay operation as illustrated in step 262.The latter determination is performed by determining, whether a CTS orCCTS message has been received when the embodiment of FIG. 4 is atissue. However, the ways to find the determinations in step 262 and 260can be performed as well. When step 262 or step 260 results in the factthat a relay operation is not necessitated, the relay apparatus does notstore the data message as indicated at 264 and 266. When however, it isdetermined that there is a CCTS message and that the relay improves thecommunication between source and destination, the wireless relayapparatus actually receives and stores the source data message asindicated in step 268. Finally, when the relay apparatus is the selectedrelay candidate, the data stored in step 268 is transmitted to thedestination apparatus as indicated at 270. Relay operation can be asimple forward operation where the data message is stored and sent outas having been stored before. When however, the relay operation is amore advanced relay operation, than the relay apparatus will process thestored source data message to generate a different relay data message.The latter functionality is better with respect to the bit error rate,but has more delay and complexity compared to a simple repeater relaymode.

FIG. 2 e illustrates a wireless relay apparatus for receiving a sourcedata message and for transmitting a relay data message based on thesource data message. Again, the wireless relay apparatus illustrated inFIG. 2 e illustrates a transmitter and a receiver section and acontroller configured for interpreting messages received by the receiversection and configured for performing a wireless relay apparatuscontention procedure when receiving an ECR message from a wirelesssource apparatus, from which the source data message originates asindicated in step 280. In step 282 a contention process feature isinitiated in response to a reception of the ECR message. It is a featurethat the ECR message starting the contention procedure comes from thewireless source device rather than the wireless destination device,since a useful relay operation has, as a precondition, that the channelbetween the source and the relay candidate is good enough so that asuccessful transmission of the data message to the relay candidate canbe expected. Thus, it is made sure that when the ECR message is notsuccessfully received by a wireless node, this wireless node will notparticipate in any relay contention procedure. In step 284, a contentionprocedure is performed by the controller in the wireless relayapparatus, which is based on probabilistic transmission. Then, in step286, the controller performs a selection of a transmission resource froma plurality of different transmission resources in accordance with thecontention procedure. The transmission resource is used for transmittingan AFR message or for transmitting a relay data message itself in caseof a protocol which is different from the FIG. 4 protocol. This isindicated at step 288 in FIG. 2 e. Features of this procedure are thatthe relay contention phase is triggered by a message from the sourceapparatus such as the ECR message. A further feature is that thewireless destination apparatus performs a channel reservation by an NACKmessage and that the source device performs a channel reservation by theECR message so that both cooperating devices perform a channelreservation in the neighborhood of both devices.

FIG. 2 f illustrates a contention protocol which can be implemented instep 286. In step 286 a a contention phase trigger information isreceived such as an ECR message from the source or an NACK message fromthe destination. The ECR message from the source is advantageous as thetrigger information. In step 286 b the controller is configured forselecting a random or pseudo-random information. In step 286 c, one of aplurality of given different transmission resources is selected, andfinally in step 286 d, the AFR message or the data message istransmitted in the selected transmission resource.

The transmission resource can be, for example, a time slot, a frequencychannel, a CDMA slot or a spatial channel. Exemplarily, there can be thesituation that there do exist six transmission slots. In this example,the random information or pseudo-random information would be a numberbetween one and six, and in response to the selected random number, acorresponding time slot, for example, is selected in step 286 c. If forexample, the time slots are numbered from one to six, then thedetermination of the random number to be three will result in theselection of the third type of slot. This procedure, which isindividually performed in each wireless relay candidate provides aminimum number of collisions in the relay contention phase, although allpotential relay candidates of course, are not synchronized to eachother, and in fact, even do not know about each other. In view of that,it is advantageous that the relay contention protocol is permanently orfixedly stored in each participating wireless apparatus.

Potential relays need to have successfully overheard the message fromthe source and have received the NACK message from the destination.After the NACK, there is a fixed number of slots where these relays sendan AFR message with a given probability ρ in each slot. A relayselection is successful, if there is at least one slot where exactly onerelay has sent its AFR. The transmission probability ρ is chosen in away to maximize the expected number of slots with exactly one AFRmessage.

The probability p that out of m potential relays only one sends an AFRin a given slot is thus given by

p−mρ(1−ρ)^(m-1).   (1)

In order to maximize p we solve

$\frac{p}{\rho} = 0$

yielding

$\rho = {{\frac{1}{m}\mspace{14mu} {and}\mspace{14mu} p_{\max}} = \left( {1 - \frac{1}{m}} \right)^{m - 1}}$

as solution. Performing a relay selection over s slots gives a successprobability p_(s) of

p _(s)=1−(1−p _(max))^(s).   (2)

The number of contention slots s determines the success probably p_(s).Choosing a high s increases the time overhead of the selection process.For up to 100 relaying candidates an s of 5 yields a p_(s) above 90%which turns out to be a good compromise between overhead for relayselection and success rate. After the reception of one or morenon-colliding AFR messages, the destination selects the best relaycandidate in terms of highest received SNR in its SFR message. In casethe relay selection fails, the absence of the destination's SFR tellsthe source to continue with a retransmission attempt for the directtransmission.

The success of a transmission is a random event with the received SNRbeing a parameter. By looking at the current link quality one canestimate the success probability of the transmission. If thisprobability for the success of a direct transmission is high, relays arelikely to be not needed and may pass on supporting the ongoingtransmission (and overhearing it). Based on an application-dependentthreshold Θ, the destination can decide at reception of the RTS whethera relay is necessitated or not (compare Relay Selection on Demand [9]).The threshold Θ specifies the PER an application can cope with. Theproposed protocol aims to keep the PER between any source destinationpair below this threshold but does not try to make the transmissions asreliable as possible at the expense of the throughput. Furthermore, ifcooperation is necessitated, potential relaying nodes assess their linkqualities to source and destination. Only nodes which can provide anoverall PER from source to the destination which is smaller than the PERof the direct channel should be considered and should consume energyoverhearing the direct transmission.

For simplicity and without loss of generality we assume BPSK withoutchannel coding in the following. A received symbol y_(i) is given by

y _(i) =h _(i) ·x _(i) +n _(i),   (3)

where x_(i) is the transmitted symbol, h_(i) is the fading coefficientwhich is Rayleigh distributed with parameter √{square root over (L/2)}and n_(i) is additive white gaussian noise with parameter N₀. The valueL represents the path loss of the observed link and N₀ is the spectralnoise density. In the case of quasi-static fading, the fadingcoefficient h_(i) is constant for the transmission of one packet. TheBER is given by [3]

$\begin{matrix}{{{BER} = {0.5 \cdot {{erfc}\left( \sqrt{\frac{h_{i}^{2}E_{b}}{N_{0}}} \right)}}},} & (4)\end{matrix}$

with E_(b) being the energy per bit at transmitter side. For uncodedmessages with j bits per packet calculation of the PER isstraightforward and given by

PER=1−(1−BER)^(j).   (5)

The PER of the source-relay and relay-destination channel is thenobtained as

PER_(SRD)=1−(1−PER_(SR))(1−PER_(RD))   (6)

where PER_(SR) and PER_(RD) are the PER for the source-relay and therelay-destination message transmissions, respectively.

Whenever the direct channel between source and destination provides aPER which is lower than the specified threshold value Θ, a regular CTSmessage is used to inform source and neighbors that no cooperation isnecessitated. In all other cases the destination uses a CCTS whichadditionally contains the current SNR of the direct link.

Potential relaying candidates determine the PER of the direct link byusing (5) with the SNR information obtained from CCTS and control theirown capability to improve the reliability of this transmission byevaluating (6) using their own link qualities measured by reception ofRTS and CCTS packets. If the expected PER of the given relay channel isbelow the expected PER of the direct channel, the corresponding relaywill listen to the transmission of the source.

The overall concepts have been implemented in a protocol named CoRe-MACand evaluated using the wireless sensor network simulator JProwler [4].

In the simulation, all control messages such as RTS and CTS as well asdata messages with different length are accounted for. Control messagesare assumed to be transmitted with half of the transmission rate of datapackets and thus experience a lower BER than the transmission of thedata messages. The simulation setup features a dedicated pair of asource and a destination node (one hop), with potential relays beingdistributed around them uniformly randomly with a given density. Thedistance between source node S and destination node D (SD-distance) isvaried during an experiment run in order to infer about the protocol'sperformance at various signal qualities. The experiments are repeatedwith different deployments of the potential relay nodes until thesimulation results show an accuracy of ±1% within a 95% confidenceinterval.

If not stated otherwise, the settings summarized in FIG. 6 are used inthe simulations. The node density is assumed to be known by the nodesand is used to predict the number of potential relays (parameter m) forthe relay selection process. The node density is currently aconfiguration parameter that needs to be set at deployment. However, thealgorithm is robust against mispredictions of this value, so anapproximate node density is sufficient.

Relay Candidates: FIG. 7 shows the number of relay candidates averagedover all transmission attempts of the source (cooperative andnon-cooperative ones) and thus reflects the additional amount of datamessage receptions due to cooperation. If the PER of the SD-channel isabove the Θ threshold, relays will not overhear the following datatransmission. In the case of a small Θ, relays are often set to overhearmessages, which comes at the cost of energy consumption for the relayingnodes while listening or the non-availability of these nodes to othercommunications while they are listening. On the other hand, a high Θincreases the chance that relaying was not activated when a directtransmission fails. For Θ=1, a protocol without relaying is obtained.The higher the distance and the lower Θ, the more relays are activatedto listen in average. For large distances, this number decreases due tothe lower number of potential relays.

Throughput: FIG. 8 compares the throughput gains of CoRe-MAC to standardCSMA/CA for different Θ values. If the SD-distance is small, theprobability for lost or incomplete messages is comparably low andCoRe-MAC operates mostly in the mode without relaying. Thus theperformance of both protocols is similar if the SNR between source anddestination is good.

With increasing distance more transmissions fail. Here, the standardCSMA/CA protocol tries to overcome this with retransmissions whileCoRe-MAC uses alternative paths via relaying. Due to the coherence ofthe channel state, retransmissions on the direct channel tend to failmore likely than relayed transmissions, which leads to a significantlybetter throughput for CoRe-MAC.

For very low SNR, direct transmissions fail very often while therelaying protocol is still able to provide communication. In this case,the relaying protocol is invoked very often and the relay behaves as anadditional hop between source and destination.

PER: FIG. 9 a-9 c illustrate the PER, i.e., the ratio of the number ofmessages not received to the number of messages originated by thesource, as function of different parameters. A message transfer isconsidered successful, if its transmission succeeds, with or without thehelp of relaying operations or retransmissions. In FIG. 8 the PER isdepicted as function of the SD-distance. As expected, the PER increaseswith the distance. However, the relaying protocol is able to provide alower PER, especially at large distances. Again, there is a tradeoffbetween Θ as activation threshold for the relays and the resulting PER.

FIG. 9 e shows the dependence of PER to the length of the data messagefor distance of 2 m between source and destination. With increasingmessage size the PER for our protocol as well as for the CSMA/CAprotocol increases. This is on the one hand a result of the decreasingPER for smaller messages and on the other hand is influenced by thecoherence time of the channel—for larger messages it is less likely thatthe channel state is coherent when doing a retransmission. Cooperativerelaying is more important if the message size is large. The selectionof the appropriate Θ value also depends on the message size.

The influence of the coherence time of the channel on the PER isillustrated in FIG. 9 c. With increasing coherence time the PERs of theinvestigated protocols decrease. However, CoRe-MAC outperforms CSMA/CAfor the whole range of considered coherence times. For small coherencetime values, the channel changes considerably between channelreservation and data transmission, resulting in a higher PER.

Average Delay: Since CoRe-MAC performs the relay selection only after afailure of the direct transmission we expect no extra overhead in thedata transmission time in comparison to standard CSMA/CA. As criteria weuse the delay, defined as the time from the start of a data transmissionat the source until successful reception of the whole data at thedestination. It contains signaling overhead, retransmissions, andcooperative transmissions. FIG. 10 depicts the average delay of thedifferent protocols (with two different settings for Θ) and supportsaforementioned hypothesis. The relaying protocol is able to reduce thedelay in case the direct transmission fails, since on average it needsless time in relaying the message than the standard protocol needs formultiple retransmissions. For large distances, however, the averagetransmission time of CSMA/CA is shorter than with relaying. This is dueto the case that at these distances most direct transmissions fail anddo not enter the statistic. The few that are received witness a goodchannel condition and thus have a shorter delay than most relayedcommunications.

The invention features a new approach for cooperative relaying based onmobile devices executing the CoRe-MAC protocol, a MAC protocol forcooperative relaying which builds on the IEEE 802.11 mechanisms. Specialattention was paid to the feasibility of the protocol forstate-of-the-art hardware and to the evaluation of its performance underrealistic assumptions.

The protocol extends the IEEE 802.11 mechanisms for handlingtransmission failures by space/time diverse channels. In the case thedirect transmission is successful, however, our protocol comes with noadditional overhead for the relay selection. Thus, for good SNR betweensource and destination CoRe-MAC has similar performance as the standardCSMA/CA approach. In the case of a transmission failure, e. g., due tosmall scale fading, our approach supports transmission via a differentcommunication path implementing spatial diversity via a selected relay.Thus, especially for transmission over unreliable communication links,the throughput, the delay, and reliability of wireless communication canbe improved.

In contrast to existing approaches, the presented approach is innovativein the following aspects: (1) Relay devices do only activate theoverhearing of signals in case of weak signal quality between sender andreceiver. (2) The selection of relay devices is done on demand only. (3)A specific process (protocol) for the reservation of the wireless mediumfor the entire cooperative communication has been specified.

Although some aspects have been described in the context of anapparatus, it is clear that these aspects also represent a descriptionof the corresponding method, where a block or device corresponds to amethod step or a feature of a method step. Analogously, aspectsdescribed in the context of a method step also represent a descriptionof a corresponding block or item or feature of a correspondingapparatus.

Depending on certain implementation requirements, embodiments of theinvention can be implemented in hardware or in software. Theimplementation can be performed using a digital storage medium, forexample a floppy disk, a DVD, a CD, a ROM, a PROM, an EPROM, an EEPROMor a FLASH memory, having electronically readable control signals storedthereon, which cooperate (or are capable of cooperating) with aprogrammable computer system such that the respective method isperformed.

Some embodiments according to the invention comprise a data carrierhaving electronically readable control signals, which are capable ofcooperating with a programmable computer system, such that one of themethods described herein is performed.

Generally, embodiments of the present invention can be implemented as acomputer program product with a program code, the program code beingoperative for performing one of the methods when the computer programproduct runs on a computer. The program code may for example be storedon a machine readable carrier.

Other embodiments comprise the computer program for performing one ofthe methods described herein, stored on a machine readable carrier.

In other words, an embodiment of the inventive method is, therefore, acomputer program having a program code for performing one of the methodsdescribed herein, when the computer program runs on a computer.

A further embodiment of the inventive methods is, therefore, a datacarrier (or a digital storage medium, or a computer-readable medium)comprising, recorded thereon, the computer program for performing one ofthe methods described herein.

A further embodiment of the inventive method is, therefore, a datastream or a sequence of signals representing the computer program forperforming one of the methods described herein. The data stream or thesequence of signals may for example be configured to be transferred viaa data communication connection, for example via the Internet.

A further embodiment comprises a processing means, for example acomputer, or a programmable logic device, configured to or adapted toperform one of the methods described herein.

A further embodiment comprises a computer having installed thereon thecomputer program for performing one of the methods described herein.

In some embodiments, a programmable logic device (for example a fieldprogrammable gate array) may be used to perform some or all of thefunctionalities of the methods described herein. In some embodiments, afield programmable gate array may cooperate with a microprocessor inorder to perform one of the methods described herein. Generally, themethods are performed by any hardware apparatus.

While this invention has been described in terms of several advantageousembodiments, there are alterations, permutations, and equivalents whichfall within the scope of this invention. It should also be noted thatthere are many alternative ways of implementing the methods andcompositions of the present invention. It is therefore intended that thefollowing appended claims be interpreted as including all suchalterations, permutations, and equivalents as fall within the truespirit and scope of the present invention.

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1. Wireless relay apparatus for receiving a source data message and fortransmitting a relay data message based on the source data message,comprising: a transmitter section and a receiver section; and acontroller configured for determining, whether the wireless relayapparatus will, when operating as a wireless relay apparatus, provide atransmission channel from a wireless source apparatus to a wirelessdestination apparatus via the wireless relay apparatus, which is betterthan a direct transmission channel from the wireless source apparatus tothe wireless destination apparatus or for determining, whether thewireless destination apparatus necessitates a relay operation or doesnot necessitate a relay operation; configured for storing the sourcedata message from a wireless source apparatus only when the transmissionchannel via the wireless relay apparatus is better than the directtransmission channel or when a relay operation is necessitated, and fornot storing the source data message in the other case; and configuredfor controlling the transmitter to transmit the relay data message, therelay data message being based on the stored source data message,wherein a cooperative clear to send message received comprises a channelcondition estimate on the direct transmission channel between thewireless source apparatus and the wireless destination apparatus, andwherein the controller is configured for extracting the channelcondition estimate from the cooperative clear to send message receivedto acquire a direct channel information, wherein the controller isconfigured for determining a relay channel information for thetransmission channel from the wireless source apparatus via the wirelessrelay apparatus to the wireless destination apparatus, and wherein thecontroller is configured for comparing the direct channel informationand the relay channel information to determine, whether the wirelessrelay apparatus will provide a better transmission channel.
 2. Wirelessrelay apparatus in accordance with claim 1, in which the controller isconfigured for only being ready for storing a source data messagesubsequent to the cooperative clear to send message received and for notbeing ready for storing subsequent to a received clear to send message,in which the controller is configured for actually storing the sourcedata message subsequent to the received cooperative clear to sendmessage and when the transmission channel via the wireless apparatus isbetter than the direct channel, and in which the controller isconfigured for not storing the source data when the controller is readyfor storing and when the transmission channel via the wireless relayapparatus is not better than the direct channel.
 3. A wireless relayapparatus in accordance with claim 1, in which the controller isconfigured for determining a first channel information for a channelbetween the wireless source apparatus and the wireless relay apparatusbased on a request to send message received, in which the controller isconfigured for determining a channel information for a second channelbetween the wireless relay apparatus and the wireless destinationapparatus based on a clear to send message received, and in which thecontroller is configured for determining a third channel information onthe transmission channel via the wireless relay apparatus based on thefirst channel information and on the second channel information. 4.Wireless relay apparatus in accordance with claim 1, in which thecontroller is configured for controlling the transmitter section totransmit an apply for relay message in response to reception of anextended channel reservation message from the wireless source device. 5.Wireless relay apparatus in accordance with claim 4, in which thecontroller is configured for controlling the transmitter section totransmit the relay data message only after receipt of a select for relaymessage identifying the wireless relay apparatus.
 6. Wirelessdestination apparatus for receiving a data message, comprising: atransmitter section and a receiver section; and a controller configuredfor interpreting messages received by the receiver section; configuredfor estimating a channel condition between a wireless source apparatusand the wireless destination apparatus in response to a request to sendmessage originating from the wireless source apparatus; configured forcontrolling the transmitter section to transmit a clear to send messagewhen the channel condition is good enough without a relaying operationand to transmit a cooperative clear to send message when the channelcondition is so that a relaying operation will become necessitated,wherein the controller is configured to form the cooperative clear tosend message so that a channel condition estimate is comprised in thecooperative clear to send message, wherein the controller is in a relaymode, when the cooperative clear to send message is transmitted and isin a non-relay mode, when the clear to send message is transmitted, andwherein the controller is configured to control the transmitter sectionto transmit, in the non-relay mode, an acknowledge message subsequent totransmitting a clear to send message and a successful receipt of asource data message, and to transmit, in the relay mode, a negativeacknowledge message subsequent to transmitting a cooperative clear tosend message and a non-successful receipt of the source data message. 7.Wireless destination apparatus in accordance with claim 6, in which thechannel condition estimate is a signal to noise ratio determined by areception of the request to send message.
 8. Wireless destinationapparatus in accordance with claim 6, in which the controller isconfigured for controlling the transmitter section to transmit, in therelay mode, the acknowledge message subsequent to a successful receiptof a relay data message from a wireless relay apparatus.
 9. Method ofoperating a wireless relay apparatus for receiving a source data messageand for transmitting a relay data message based on the source datamessage, comprising a transmitter section and a receiver section; and acontroller, the method comprising: determining, whether the wirelessrelay apparatus will, when operating as a wireless relay apparatus,provide a transmission channel from a wireless source apparatus to awireless destination apparatus via the wireless relay apparatus, whichis better than a direct transmission channel from the wireless sourceapparatus to the wireless destination apparatus or determining, whetherthe wireless destination apparatus necessitates a relay operation ordoes not necessitate a relay operation; storing the source data messagefrom a wireless source apparatus only when the transmission channel viathe wireless relay apparatus is better than the direct transmissionchannel or when a relay operation is necessitated, and not storing thesource data message in the other case; and controlling the transmitterto transmit the relay data message, the relay data message being basedon the stored source data message, wherein a cooperative clear to sendmessage received comprises a channel condition estimate on the directtransmission channel between the wireless source apparatus and thewireless destination apparatus, and wherein the channel conditionestimate is extracted from the cooperative clear to send messagereceived to acquire a direct channel information, wherein a relaychannel information for the transmission channel from the wirelesssource apparatus via the wireless relay apparatus to the wirelessdestination apparatus is determined, and wherein the direct channelinformation and the relay channel information are compared to determine,whether the wireless relay apparatus will provide a better transmissionchannel.
 10. Method of operating a wireless destination apparatus forreceiving a data message, comprising a transmitter section and areceiver section; and a controller, the method comprising: interpretingmessages received by the receiver section; estimating a channelcondition between a wireless source apparatus and the wirelessdestination apparatus in response to a request to send messageoriginating from the wireless source apparatus; controlling thetransmitter section to transmit a clear to send message when the channelcondition is good enough without a relaying operation and transmitting acooperative clear to send message when the channel condition is so thata relaying operation will become necessitated, wherein, when the clearto send message is transmitted, an acknowledge message is transmittedsubsequent to transmitting the clear to send message and a successfulreceipt of a source data message, and wherein, when the cooperativeclear to send message is transmitted, a negative acknowledge message istransmitted subsequent to transmitting the cooperative clear to sendmessage and a non-successful receipt of the source data message. 11.Computer program for performing, when running on a computer, the methodof operating a wireless relay apparatus for receiving a source datamessage and for transmitting a relay data message based on the sourcedata message, comprising a transmitter section and a receiver section;and a controller, the method comprising: determining, whether thewireless relay apparatus will, when operating as a wireless relayapparatus, provide a transmission channel from a wireless sourceapparatus to a wireless destination apparatus via the wireless relayapparatus, which is better than a direct transmission channel from thewireless source apparatus to the wireless destination apparatus ordetermining, whether the wireless destination apparatus necessitates arelay operation or does not necessitate a relay operation; storing thesource data message from a wireless source apparatus only when thetransmission channel via the wireless relay apparatus is better than thedirect transmission channel or when a relay operation is necessitated,and not storing the source data message in the other case; andcontrolling the transmitter to transmit the relay data message, therelay data message being based on the stored source data message,wherein a cooperative clear to send message received comprises a channelcondition estimate on the direct transmission channel between thewireless source apparatus and the wireless destination apparatus, andwherein the channel condition estimate is extracted from the cooperativeclear to send message received to acquire a direct channel information,wherein a relay channel information for the transmission channel fromthe wireless source apparatus via the wireless relay apparatus to thewireless destination apparatus is determined, and wherein the directchannel information and the relay channel information are compared todetermine, whether the wireless relay apparatus will provide a bettertransmission channel.
 12. Computer program for performing, when runningon a computer, the method of operating a wireless destination apparatusfor receiving a data message, comprising a transmitter section and areceiver section; and a controller, the method comprising: interpretingmessages received by the receiver section; estimating a channelcondition between a wireless source apparatus and the wirelessdestination apparatus in response to a request to send messageoriginating from the wireless source apparatus; controlling thetransmitter section to transmit a clear to send message when the channelcondition is good enough without a relaying operation and transmitting acooperative clear to send message when the channel condition is so thata relaying operation will become necessitated, wherein, when the clearto send message is transmitted, an acknowledge message is transmittedsubsequent to transmitting the clear to send message and a successfulreceipt of a source data message, and wherein, when the cooperativeclear to send message is transmitted, a negative acknowledge message istransmitted subsequent to transmitting the cooperative clear to sendmessage and a non-successful receipt of the source data message.